Risk allocation in a freshwater gastropod.

To balance the basic needs of organisms, internal and external cues are used to inform the optimal behavioral strategy. Some of the best-studied related cognitive rules have emerged in predator-prey contexts, such as the threat-sensitivity hypothesis, which postulates that prey should adjust their antipredator behavior in accordance with the level of risk. Extending this theory, the risk allocation hypothesis posits that under long-term sustained high predation risk, individuals should decrease their antipredator responses towards risky stimuli so as to meet their energetic demands. Evidence for the risk allocation hypothesis has been mixed in invertebrates, particularly in gastropods, which are classic model systems for antipredator responses. This may be due to past studies frequently lacking sham controls and/or sufficient certainty about the risk regime. The present study in the aquatic gastropod Physella acuta controls for these factors by crossing long-term background risk, ie lifelong consistent exposure to conspecific alarm cues, which reliably signal high predation risk (high-risk), or a water control (low-risk) with exposure to a high-risk or low-risk stimulus. Crawl-out behavior is an adaptive antipredator response in gastropods. In accordance with threat-sensitivity, high-risk stimuli induced increased crawl-out behavior independent of background risk. Providing partial support for risk allocation, high background risk induced lower responsivity to both low-risk and high-risk chemical stimuli. This may be because cue addition also provided tactile cues that could be considered risky by high background risk snails. Altogether, the present well-controlled research contributes novel data to the hitherto mixed evidence for risk allocation in gastropods.